Nonlinear optical switching in lithiated cyclo[6]carbon: external-electric-field controlled dominance and spectral...

Precise modulation of nonlinear optical (NLO) properties in functional materials is crucial for advancing photonic and optoelectronic technologies. Herein, we theoretically designed two stable monolithiated-C₆ configurations (Li@C₆ complexes) with distinct Li positions on the cyclo[6]carbon ring. These configurations exhibit facile room-temperature interconversion due to low free energy barriers (2.37-7.22 kcal/mol) and high-rate constants, with reversible dominance under external electric fields (EEFs). Electronic structure, absorption spectra, and NLO analyses demonstrate that Li positioning governs optical behavior. Li@C₆^OUT displays enhanced absorption intensity, broader spectral range, and stronger Li-directed charge-transfer characteristics compared to Li@C₆^TOPIN. Remarkably, EEF application amplifies first- and second-order hyperpolarizabilities by factors of 16.5/20.4 (Li@C₆^OUT) and 12.4/15.5 (Li@C₆^TOPIN), respectively, while polarizability remains stable. Electron density difference, hole-electron distributions, and hyperpolarizability density analyses corroborate these trends. The pronounced EEF-responsive electronic and NLO variations in Li@C₆ complexes highlight their promise as optically switchable molecules, offering a design strategy for tunable molecular photonic devices.